High frequency energy interchange device



Jan. 19, 1960 R. M. VAN DIEN 2,922,067

HIGH FREQUENCY ENERGY INTERCHANGE DEVICE Filed Oct. 29, 1958 2Sheets-Sheet 1 INVENTOR.

A TTOE/l/E Y o I a m m U H M O H I N I g I m m H I N I] I H POLAND M.VANDIEN Jan. 19, 1960 R. M. VAN DIEN HIGH FREQUENCY ENERGY INTERCHANGEDEVICE Filed Oct. 29. 1958 2 Sheets-Sheet 2 POLAND M. VAN DIEN ATTORNEYcuit elements.

United States Patent i HIGH FREQUENCY ENERGY INTERCHANGE DEVICE RolandM. Van Dien, Menlo Park, Califi, assignor to General Electric Company, acorporation of New York Application October 29, 1958, Serial No. 770,367

3 Claims. (Cl. 3153.5)

This invention relates to high frequency energy interchange deviceswherein the interchange of energy takes place between a stream ofelectrons and a radio frequency field to provide amplification and/ oroscillations. More particularly, the invention relates to the class ofhigh frequency energy interchange devices known as traveling-wave tubes,which include an electron gun for producing a stream of electrons in aninteraction region and a radio frequency circuit or transmission linefor producing radio frequency fields in the region of interaction; andthe invention has for one of its principal objects the provision ofattenuators for such devices which allow improved performance andsimplification of design and construction.

The present invention is particularly applicable to traveling-wave tubeswhich utilize a slow-wave radio frequency circuit of the helix orhelix-derived type. It is intended that each of the terms helix (orhelical) and helix-derived be applied to radio frequency slow-wavetransmission lines in the manner commonly used in the traveling-wavetube art. For example, the terms encompass wire and tape helices of thesingle and multifilar varieties and the cross-wound or contra-woundstructures. The terms also encompass the circuit of the so-calledring-bar types. Such helix and helix-derived slow-wave circuits areillustrated and described in many texts and articles. For example, seeM. Chodorow and E. L. Chu, Cross-Wound Twin Helices for Traveling-WaveTubes, Journal of'Applied Physics, vol. 26, pp. 3334, January 1955;Samuel Sensiper, Electromagnetic Wave Propagation on Helical Structures,Proceedings of the IRE, vol. 43, pp. 149-161, February 1955; and C. K.Birdsall and T. E. Everhart, Modified Contra-Wound Helix Circuits forHigh-Power Traveling-Wave Tubes, IRE Transactions on Electron Devices,vol. ED-3, No. 4, pp. 190-204, October 1956.

It is very important in traveling-wave tubes using helical slow-wavecircuits that the helix be supported firmly and accurately inpredetermined position with respect to the electron stream and theexternal radio frequency cir- In addition, when there is appreciablepower flowing down the helix, or when there is some interception of theelectron stream by the circuit, a means of dissipating heat produced inthe helix turns must be provided. Commonly, both the mechanical supportand heat-dissipating requirements are met by supporting the helix on oneor more rods of a dielectric material such as synthetic sapphire havinghigh mechanical strength and a high thermal conductivity. Thisarrangement however, is not acceptable in all situations and isundesirable for most situations since the electrical performance (rateof gain, efi'iciency, etc.) of the helix as a circuit element isdegraded by the presence of the support rods. That is to say that theportion of the dielectric support rods between each pair of turns of thehelix is effectively a capacitance connected between the turns whichreduces the impedance of the helix and consequently reduces theeffectiveness of the helix in producing 2,9Z2fi67 Patented Jan. 19, 1960HQC an interaction between electromagnetic waves propagated therealongand electrons in an electron stream in interacting relationship with theelectromagnetic waves. This effect of the dielectric rods is calleddielectric loading. Accordingly, it is an object of the presentinvention to provide a sound and accurate means for mounting the helixin a traveling-wave tube which is also capable of dissipating heatwithout seriously degrading the operation of the helix by contributingto the dielectric loading.

Attempts to solve the problem outlined above have included the use ofvery small solid rods having a low dielectric constant to support thehelix in a travelingwave tube. Such a solution has not proved to besatisfactory principally due to the heat transfer problem. The side ofthe supporting rods away from the helical circuit must be in thermalcontact with a mass of high heat conductivity such as a metallic blockor envelope. Since the metallic surface cannot be placed close to thehelix for electrical reasons, there is a minimum acceptable support roddiameter. Furthermore, in choosing the dielectric constant of thesupport rod, one is limited to material having relatively high thermalconductivities. For example, the most obvious choice is syntheticsapphire which has a relatively high dielectric constant where epermittivity and s is the permittivity of free space).

Accordingly, it is a further object of this invention to provide asupport structure for the helix of a travelingwave tube which meets themechanical and heat dissipating requirements and which does notnecessitate making the traveling-wave tube excessively long in order tocompensate for dielectric loading due to the support.

In carrying out the objects of the present invention, a traveling-Wavetube is provided which has a slow-wave structure of a helical orhelix-derived form and one or more support members for the slow-wavestructure which support members for the slow-wave structure arecontoured to present essentially a line contact down the length of theentire slow-wave structure.

The novel features which are believed to be characteristic of theinvention are set forth in the appended claims. The invention itself,however, both as to its organization and method of operation, togetherwith further objects and advantages thereof, may best be understood byreference to the following description taken in connection with theaccompanying drawings in which:

Figure 1 is a broken and partially broken away side elevation of a highfrequency energy interchange device embodying the present invention;

Figure 2 is a transverse section through the tubular body portion of thedevice of Figure 1 taken on line 22 of Figure 1;

Figure 3 is a perspective view of a segment of the slowwave transmissionline used in the device of Figure Figure 4 is a perspective view ofanother type of helical slow-wave structure and its support which may beutilized with the traveling-wave tube illustrated in Figures 1 and 2;

Figure 5 is a transverse section through the tubular body of atraveling-Wave tube illustrating how the helix and helix support ofFigure 4 may be positioned within the tubular body; and

Figure 6 is a transverse section through the tubular body of atraveling-wave tube similar to that of Figure 5 illustrating anothermeans of using the helix supports of the present invention to support ahelix in the tubular body of a traveling-Wave tube.

In Figure 1 of the drawings, a traveling-wave tube 10 is illustrated.Traveling-wave tube 10 is provided with an envelope '11 which isgenerally long and cylindrical.

As illustrated, the envelope 11 has an enlarged cylindrical glass andmetal portion 12 at one end and a smaller longer metallic cylindricalportion 13 forming the opposite end. The enlarged cyclindrical portion12 houses an electron stream producing gun 18 and the small 5 slendercylindrical portion 13 houses the long helical main interactiontransmission line 14. Thus the two principal interaction producingelements are enclosed in the envelope 11. That is, electrons areprojected down the length of the slow-wave circuit 14 by electron gun 18in such a manner that they interact with eletromagnetic waves propagateddown the slow-wave transmission line 14 to produce amplification. Thisphenomenon is described in more detail below.

The metal and glass portions of the electron gun enclosing bulb 12 aresealed to form a unitary enclosure which will hold a vacuum. The metalfront end of the gun enclosing bulb 12 is positioned coaxially withrespect to the metal helix enclosing cylindrical portion 13 of thevacuum tube such as by brazing. The particular main transmission line 14illustrated (best seen in Figures 1 and 3) is one of the helix-derivedslow-wave circuits which is a contra-wound helix of the type known as aringbar structure. The particular type of transmission line is calledhelix-derived since it does not conform to the strict technicaldefinition ofa helix and yet is a circuit which performs (electrically)like a contra-wound helix. The ring-bar circuit is formed by a series ofidentical coaxial rings 15 which are spaced apart in the axialdirection. Each of the rings 15 is joined to the next succeeding ringwith short bars 16 and each bar 16 is spaced A stream of electrons isproduced and projected along the axis of the envelope 11 by the electrongun 18 as depicted by the broken lines 19 in Figure 1. The electrons 4which are formed into a stream are emitted from a cathode 20 in responseto heat applied thereto by a heater member 21. As illustrated, theheater member 21 is a high resistance coil located near the emittingcathode button 20. The stream 19 is formed and projected along amplitudeof the wave on the helix 14 grows exponentially the longitudinal axis ofthe tube by a centrally apertured electron stream focusing electrode 22and a centrally ap'e'rtured electron stream accelerating anode 23. Sincethe elongated main slow-wave transmission line 14 is positioned insidethe slender portion 13 of the envelope with its axis coincident with thetube axis, the electron stream 19 is projected down the length of thetube in close proximity thereto, thus providing an interaction regionalong the axis of the tube. A collector anode 24 is positioned at theopposite or output end of the structure to dissipate residual energy inthe stream. The collector anode 24 is provided with a series of annularcooling fins 26 therearound.

In order to prevent the space charge of the electrons from spreadingthem to such an extent that they pass out This is typically done byproviding a long annular solenoid 25 which surrounds the entire tubealong its length. To simplify the present drawing and description, themagnetic field producing solenoid 25 is only partially and schematicallyillustrated and its source of-energizing potential is not .shown.

low in amplitude.

Further, the operating power dielectric, that part on either side of theslot, istin source for the tube 10 is not shown since it is conventionaland does not form a part of the present invention. The potentials on theelectrodes of the electron gun are established by means of conductiveleads or pins 26 which are brought out through the base 12 of the tube10. The relative potentials applied to the various electrodes areestablished in accordance with well known gun design considerations (seeI. R. Pierce Theory and Design of Electron Beams, 2nd edition, 1954, Van Nostrand Co.)

A desired radio frequency electromagnetic wave is coupled or transferredonto the slow-Wave transmission line 14 by a coaxial transmission line29 in such a manner that the transferred wave is propagated from the gunend of the tube toward the output or collector end. In order toaccomplish this, the inner conductor 30 of the coaxial transmission line29 is brought into the envelope 11 near the gun end of the tube andelectrically connected with a bar 16 (cross-over point of two rings ofthe slow-wave transmission line 14), and the outer conductor 31 isconnected to ground or reference potential. In the embodimentillustrated, the outer conductor 31 is connected to the cylindricalmetal helix enclosurej13. Since the coaxial transmission line 29 isbrought-into the vacuum tube through the outer wall, a dielectric vacuumtight seal or window 32 is provided between the inner and outerconductors to prevent tube leakage.

Radio frequency energy is abstractedfrom the tube 10 near the collectorend by an output coaxial transmission line .33 which also extends outthrough the metal helix enclosure 13. The output coaxial transmissionline 33 also has an inner conductor 34 which makes contact with .a bar16 or cross-over point of two rings 15 and an outer conductor or cablesheath .35 connected to the metallic helix enclosure 13 which is atground or-reference potential. Once againit is necessary to provide adielectric seal or window between the inner and outer conductors 34 and35 in order to prevent tube leakage. The details of the outputconnections and window are not-shown since they are identical to theconnection-and window 32 for input transmission line 29.

The apparatus described thusfar provides conventional traveling-waveinteraction and the particular. apparatus illustrated acts as anamplifier. That is to say, the electromagnetic waves introduced onto thehelix 14 by the input transmission line 29 interact with the electronsin the electron stream 19 to produce electron velocity modulation andconsequently electron bunching. .As the wave and stream travel along thelength of the slow-wave transmission line 14, the phenomenon reversesand the bunched stream induces fields and currents along the helix. The

until the stream becomes saturated because the stream gives up moreenergy to the helix 14 than it abstracts from it. However, it isapparent that the interaction cannot take place in an efficient mannerunless'the helical winding 14 is firmly and accuratelysupported incoaxial alignment with the electron stream. In the apparatus illustratedin Figures 1 and 2, a sound mounting for the helix 14 is provided bythree heat conductive and electrically non-conducting dielectric supportrods 42, preferably a synthetic sapphire, spaced equidistant around theperiph ery and extending the full length of transmission line 14. Theside of each rod 42 which is-in contact with the helix 14 is contouredor provided with a slot 39 down its full length. 'In this manner,contact between each support rod 42 and the helix 14 is essentially aline contact on two spaced apart lines defined by the two knife edges 40formed by the slot 39. In other words, each helix turn is supported byeach rod 42 at two points where it contacts the edges of the slot 39. a

The slot 39 in each supportrod 42 is purposely made deep enough that itsbottom is-in a-region Where radio frequency electric fieldsestablished-on the helix 14 are Thus, only a small portion of the theregion where the electric fields are high in amplitude. The practicalresult of this arrangement is that an absolute minimum of dielectricloading of the helix 14 is provided by the support rods 42 andconsequently an absolute minimum of degradation in performance of thehelical circuit results. The helix turns may be brazed to the supportrods 42 although it is not necessary with the arrangement illustrated inFigures 1 and 2.

The outer sides of the support rods 42 are in intimate contact with theinner surface of the metallic envelope 11 and thus provide the necessarysupport for the helix 14. This arrangement also provides excellentcooling for the helix, since the sapphire support rods 42 are good heatconductors and the metallic envelope 11 acts as a heat sink. Heatconduction between the support rods 42 and metallic envelope 11 can beimproved somewhat by flattening the outer sides of the rods to enlargethe area of contact between the envelope 11 and rods 39 but this is notusually necessary. Also, the sapphire rods 42 may be brazed to the innersurface of the envelope 11- to provide additional rigidity.

Figure 4 illustrates one means of supporting a tape helical slow-wavetransmission line 43 with a contoured support rod 44 in a manner similarto that illustrated in connection with the ring-bar cross wound helix 14of Figures 1, 2 and 3. As illustrated in Figure 4, the contoured supportrod 44 is circular in cross section with a rectangular slot 45 milledout along its entire length so that two knife edges 46 are formed alongthe length of the rod. The tape helix 43 then is positioned with oneside in the milled-out slot in such a manner that each helix turn issupported at two points where it contacts the edges 46 of the slot.

Figure 5 illustrates the way in which the tape helix 43 may be supportedwithin a tube envelope 11 by the single support rod 44. In theembodiment illustrated, the turns of the helix 43 are brazed to theknife edges 46 on the support rod at their point of contact and theouter edge or periphery of the support rod 44 is brazed along the innerperiphery of the metallic envelope 11 in such a manner as to hold thetape helix 43 in a properly aligned position.

Obviously the tape helix 43 may be held in position in the envelope 11by supporting it between three support rods spaced equidistant aroundits periphery in a fashion similar to the way in which the support rods42 support the ring-bar helix 14 (best seen in Figure 2.}. Such anarrangement does not require brazing of the helix 43 to the support rods44 or the support rods 44 to the envelope 11. Further, the number ofsupport rods 44 utilized is not crucial to the invention. For example,two support rods 44 may be placed on opposite sides of the tape helix 43so that the helix is trapped between four opposing knife edges 46 andthis assembly may be inserted in an envelope 11 in such a manner as toform a snug fit Within the envelope. Such an arrangement is illustratedin Figure 6.

The arrangements illustrated show a means of supporting a helixslow-wave circuit of a traveling-wave tube which provides the desiredmechanical positioning and. rigidity of the helix, excellent cooling ofthe slow-wave structure, and a very minimum of degradation ofperformance due to dielectric loading. The extent to which theelectrical performance of the helix is improved due to the use of thecontoured support rods illustrated is indicated by the curves of Figure7.

A comparison of the dielectric loading factors for a tape helix in freespace and for a tape helix supported on contoured and un-contouredsupport rods gives an accurate indication of the improvement in helixperformance due to the use of support rods which arecontoured inaccordance with the present invention. The dielectric loading factor(DLF) is efiectively a measure of the reduction in field strength of anactual helix from that of a sheath helix due to space harmonic contentandi dielectric loading of the helix. For one tape helix of the typeillustrated in Figure '4, the dielectric loading factor in free spacewas found to be 0.86; brazed on an uncontoured sapphire rod, thedielectric loading factor was found to be 0.65; and brazed on acontoured rod, the dielectric loading factor was 0.76.

As reported by P. K. Tien in an article entitled Traveling-Wave TubeHelix Impedance, Proceedings of the IRE, vol. 41, pp. 1617-1623 November1953, the interaction impedance of a helix is approximately proportionalto the square of the DLF. Thus, a given helix brazed to a contouredsapphire support rod has an impedance of (0.76) Z or 0.57762 where Z isthe impedance of a sheath helix of corresponding size and phase velocityin free space as compared with an impedance (0.65 Z or 0.42252 for thesame helix brazed to a solid sapphire rod. Thus, a given helix brazed toa slotted sapphire support rod has a considerably higher impedance thanone brazed to a solid rod. Since the gain per unit length for a helix ina traveling-wave tube with a given electron stream is determined by theimpedance of the helix, it is apparent that the performance of a helixbrazed to a contoured sapphire rod as illustrated in Figure 4 is muchbetter than that of a helix brazed to a solid sapphire rod or a helixwhich is enclosed by dielectric material. In addition to the electricalsuperiority of the contoured support rod 42 over the uncontouredcounterpart, the contoured rod provides a secure and accurate mountingfor the helix and a good means of removing heat from the helix.

While particular embodiments of the invention have been shown, it will,of course, be understood that the invention is not limited thereto sincemany modifications, both in the circuit arrangements and in theinstrumentalities employed, may be made. It is contemplated that theappended claims will cover any such modifications as fall within thetrue spirit and scope of the invention.

What I claim is new and desire to secure by Letters Patent of the UnitedStates is:

1. A traveling-wave tube comprising an elongated evacuated envelopecontaining a source of electrons adapted to project a stream ofelectrons along the longitudinal axis of said envelope, a helicalslow-wave transmission line positioned coaxially with the longitudinalaxis of said envelope, and at least one elongated insulating support rodpositioned adjacent an inner wall of said envelope, said elongated rodbeing of heat conductive dielectric material contoured to provide adepressed portion and a pair of edges extending longitudinally of saidrod, said edges being spaced apart to receive and support said helicaltransmission line at two points on each turn and the contour of said rodbeing such that the depressed portion is remote from high amplitudeelectric fields from said transmission line.

2. In combination in a traveling-wave tube having an elongated evacuatedenvelope, means to form and direct :a stream of charged particles downthe length of the envelope, a helical slow-wave transmission linepositioned coaxially with said elongated envelope, and at least oneelongated insulating support rod of circular cross section positionedadjacent the inner wall of the envelope, .said elongated rod being ofheat conductive dielectric material and provided with a longitudinallyextending groove diametrically opposite the line of contact with :theenvelope wall defining a pair of spaced apart helix supporting edgeswhich receive and support said helical transmission line and a bottomportion sufiiciently removed from said supporting edges to insure thatelectric line positioned eoaggia lly with the longitudinal axis of saiden e epe, an two 9 u e e ongated insulat ng uP- port rods positionedadjacent inner walls of said envelope, said elongated rods being of heatconductive dielectric mat a sonto red t Pr id a depressed Portion and apair of edges extending longitudinally of eaeh of said rods, said edgesbeing spaced apart to receive and support said heligal flansmission lineat two points on eaeh turn and the contour of said rod being sueh thatthe depressed portion is remote from higli' amplitude 8 electric fieldsfrom said transmission line, said insulating support rods beingpositioned around the inner wall of said'fenvelope to hold said heli'caltransmission line in co pression the et ReferencesCited in the tile ofthis, patent UNITED STATES PATENTS 1,616,184 Griflith 'Feb. 1, 19271,720,443 Robinson July 9, 1929- 0 1,812,499 Robertson Nov. 5, 195,?

